OpenWRT is a third party firmware available for the Linksys WRT54G and similar units. It is a joint effort licenced under the GPL, so it is free for everyone to use and contribute. From the (http://www.openwrt.org OpenWRT site):
OpenWrt is a linux distribution for the Linksys WRT54G. Instead of trying to cram every possible feature into one firmware, OpenWrt provides only a minimal firmware with support for add-on packages. For users this means the ability to custom tune features, removing unwanted packages to make room for other packages and for developers this means being able to focus on packages without having to test and release an entire firmware.
(http://en.wikipedia.org/wiki/OpenWrt http://en.wikipedia.org/wiki/OpenWrt)